Previous computer input devices, such as mice, include rotatable balls mounted within a housing, yet rotatably engaging a surface. As the housing of such a mouse translates across the surface, the ball rotates within the housing, engaging horizontal and vertical wheels that rotate against the ball, thereby indicating horizontal and vertical movement of the mouse across the surface. When the device is lifted from the surface, hereinafter referred to as lift-off, the ball stops rotating and the horizontal and vertical movement information provided by the wheels stops. Horizontal and vertical wheel rotation translates into an on-screen visual image of a cursor that responds to movement of the device. Because such devices have a moving ball that must pass through a hole in the housing, such devices often become contaminated with dust and dirt, which may yield inaccurate or intermittent cursor tracking. Moreover, the tracking surface and ball must have sufficient friction between the two to cause the ball to rotate when the housing translates over the surface. To help provide such friction and minimize contamination of the device, specialized tracking surfaces (e.g., mouse pads) are typically used. Thus, a major limitation of such a device is that it requires a tracking surface with particular characteristics, such as adequate friction and cleanliness, which are not readily found on all surfaces.
Building upon these primarily mechanical tracking devices, optical tracking devices have become available. Such devices optically track movement of a surface, rather than mechanically as with the devices described immediately above. These systems may avoid some of the drawbacks associated with the devices described above. In particular, these devices typically do not require wheels in contact with a movable ball, which acts as a common collection point for dust and dirt. Instead, the ball is typically covered with a distinct pattern. As the ball rotates over a surface, photodetectors facing another side of the ball collect information about the movement of the distinct pattern of the ball as the ball rotates. A tracking engine then collects this information, determines which way the pattern is translating and translates a cursor similarly, as described above. These devices offer improvements over previous designs by eliminating moving parts (the wheels) and changing the ball detection interaction from mechanical to optical. However, such devices lack the ability to track on any surface, requiring a suitable frictional interface between the ball and the surface. Moreover, these devices still require one moving part, the ball. In addition, aliasing artifacts may cause the cursor to skip, rather than move fluidly.
Still other optical devices place the pattern on the tracking surface (e.g., a mouse pad), rather than on the rotatable ball, thereby using the mouse pad to generate optical tracking information. Although such devices are able to eliminate the moving ball, they are less universal by requiring a specific tracking surface to operate.
Other more recent optical tracking devices have emerged that have eliminated the need for a patterned ball or mouse pad. One such device utilizes an LED to project light across the tracking surface at a grazing angle to the tracking surface. The mouse then collects tracking information by two methods. First, by tracking changes in color on the tracking surface by any pattern that may appear on the tracking surface, or second, by detecting dark shadows cast by high points in the surface texture, which appear as dark spots. This device eliminates the moving ball of previous devices, and is useful on a variety of surfaces. However, smooth surfaces with little color variation may prove difficult to track upon. Smooth surfaces generate no shadows from texture and present a low contrast image that may not provide adequate features upon which to track. Surfaces with very little color variation also present a low contrast image that may not provide adequate features upon which to track. For example, such a mouse could not track upon opal glass, which is a material consisting of very small crystals imbedded in a clear glass matrix. Opal glass has an opaque white surface and is very smooth.
However, these systems lack the ability to both track movement of a tracking surface over substantially any surface and detect when the device has been removed from the tracking surface for freezing the cursor.